“Electrically driven bound-exciton emission in direct-bonded silicon p-n junctions for quantum-light applications”

2	Cho, SH; Moroshkin, P; Xu, Jimmy; Nam, KT

Electrically Driven Bound-Exciton Emission in Direct-Bonded Silicon p–n Junctions for Quantum-Light Applications Sung Hoon Cho1, Petr Moroshkin2, Jimmy Xu2, and Ki Tae Nam1* 1 Department of Materials Science and Engineering, Seoul National University, Seoul, Korea 2 Division of Engineering and Department of Physics, Brown University, Providence, Rhode Island, USA *E-mail address: nkitae@snu.ac.kr Silicon is the backbone of modern microelectronics, but is inherently poor in light emission due to its indirect bandgap. Nevertheless, recent advances in defect engineering and nanoscale interface control have reopened the pathway toward achieving electroluminescence (EL) and even quantum-light emission from silicon-based materials. In this work, we report the observation of low-temperature electroluminescence originating from free and bound excitons in silicon, realized through a precisely engineered p–n junction formed by direct wafer bonding of p-type and n-type Si wafers separated by an ultrathin oxide layer. This approach demonstrates that excitonic recombination in crystalline silicon can be electrically driven in a controllable fashion, revealing bound-exciton transitions with narrow linewidths and characteristic phonon replicas. Direct-bonded silicon p-n junction was fabricated by bonding boron-doped p-type and phosphorus-doped n-type silicon wafers (resistivity 1–10 Ω·cm) after O₂ plasma surface activation, forming a 6 nm amorphous SiO₂ layer at the interface. The structure was annealed at 1050 °C in vacuum to establish covalent interfacial bonds, yielding an atomically abrupt junction without implantation-induced damage. This configuration ensures efficient carrier injection while maintaining the crystalline quality of both wafers, making it an attractive architecture for the integration of silicon-based quantum emitters. Photoluminescence (PL) measurements under 780 nm laser excitation at cryogenic temperatures (~20K) revealed a series of sharp emission features centered at 1090 nm and 1130 nm, assigned to free-exciton emission assisted by transverse acoustic (TA) and transverse/longitudinal optical (TO/LO) phonons, respectively. In addition to these dominant peaks, several weaker and narrower lines were observed, at longer wavelengths. These were attributed to excitons bound to boron impurity atoms, exhibiting distinct phonon-assisted transitions (BTA, BTO, BLO). Such bound-exciton emissions are of particular interest, as they correspond to quantum two-level systems capable of emitting single photons when only one exciton is localized at a defect center. More significantly, electroluminescence spectra at cryogenic temperatures (T ≈ 12 K) displayed analogous spectral features, confirming the electrical generation of excitonic emission. Under forward bias, the EL signal showed prominent peaks at 1129.5 nm (free exciton, FETO,LO) and 1136.5 nm (bound exciton, BTO(b1)), while weaker features near 1097 nm were attributed to bound excitons interacting with acoustic phonons (BTA(b1)). The linewidth of the bound-exciton peaks was below 1 nm, limited by the spectrometer resolution. The emission intensity exhibited a nonlinear dependence on current: increasing up to 10 mA and then diminishing beyond 20 mA due to Joule heating. This thermal suppression behavior highlights the delicate balance between carrier injection and local temperature rise in SiO2 tunneling barrier limited EL emission. Compared to prior reports of silicon EL dominated by dislocation-related or defect-cluster emission at higher temperatures (77–300 K), our results demonstrate a distinct regime of bound-exciton electroluminescence achieved through clean, direct-bonded junctions with low dopant concentrations (~10¹⁵–10¹⁶ cm⁻³). The oxide-mediated potential barrier not only limits leakage current but may also generate local electric fields and strain that stabilize bound-exciton formation. The observed blueshift of the excitonic transitions relative to literature values (~3 nm) further suggests subtle confinement effects induced by the SiO₂ interfacial layer, consistent with the creation of an effective quantum well on either side of the bonded interface. The realization of electrically driven bound-exciton emission in silicon provides a crucial step toward the long-sought goal of an all-silicon single-photon emitter operating at telecom-compatible wavelengths (~1130 nm). The emission mechanism, rooted in impurity-bound excitons, naturally provides quantum-light characteristics such as antibunching and spectral purity, while the direct-bonded diode platform ensures compatibility with existing silicon photonics and CMOS processes. Future efforts will focus on the deterministic control of impurity sites, integration with optical cavities to enhance photon extraction efficiency, and operation at elevated temperatures. In summary, this study establishes a new paradigm for defect-mediated light emission in crystalline silicon. By leveraging direct wafer bonding to form high-quality junctions and exploiting exciton localization at dopant sites, we demonstrate that silicon—traditionally considered an inefficient emitter—can be transformed into a viable platform for electrically driven quantum-light sources. This approach bridges the gap between conventional semiconductor technology and emerging quantum photonics, enabling scalable integration of single-photon emitters directly within the silicon photonic ecosystem.

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